TIMEOUT
MAYBE
Trs:
 {     f(mark(X)) -> f(X),
     f(active(X)) -> f(X),
        mark(c()) -> active(c()),
       mark(f(X)) -> active(f(X)),
       mark(g(X)) -> active(g(X)),
      active(c()) -> mark(f(g(c()))),
  active(f(g(X))) -> mark(g(X)),
       g(mark(X)) -> g(X),
     g(active(X)) -> g(X)}
 Open